Computerized tomography refers to the procedures used to generate two dimensional maps of some physical quantity in a planar section of a target by measuring and analyzing the attenuation of beams of penetrating radiation passed through the target along sets of coplanar rays. As practiced, a complete apparatus must contain four elements: (1) a source of penetrating radiation, (2) detectors that measure the transmitted intensity of the radiation after passage through the target, and that can be calibrated to give the unattenuated intensity of radiation in the absence of the target, (3) a computational device to store and process the attenuation measurements, converting them into a digital map of attenuation coefficients in the observed plane of the target, and (4) a device to display the resultant image.
Tomography can be practiced in many ways, but the broadest commercial usage is in medical radiology to provide diagnostic maps of bone and tissue structure in human patients (W. Swindell and H. H. Barett, "Computerized Tomography: Taking Sectional X-Rays", Physics Today, pp. 32-41, 1977; C. C. Jaffe, "Medical Imaging", American Scientist, 70, 576 (1982); and P. Alexander, "Array Processors in Medical Imaging", Computer, 16, (1983). Medical CT uses broad band bremsstrahlung radiation from X-ray tubes to produce penetrating radiation that is measured, typically, by scintillation crystals and photo-tubes. Measurements are stored in a programmable digital computer and analyzed using a method generically referred to as convolution (or filtered) back projection (referred to hereafter as FBP). The density map derived from the analysis is displayed on a cathode ray tube as a two dimensional cross sectional image containing approximately 250.times.250 elements or pixels, with a resolution of about 1 millimeter, and 1% accuracy in determination of X-ray attenuation coefficient. Medical procedures typically produce scans in only a limited number of adjacent body planes, say one to twenty. Other special purpose tomography probes have been built using different types of ionizing radiation such as gamma rays, and electrons.
An object of the present invention is to provide a microtomography device which uses an intense well collimated beam of radiation to produce three dimensional images with improved spatial resolution. Spatial resolution attainable using the microtomography device can be as small as 0.5 microns which is 100-1000 times better than that achieved with conventional medical CT. A further object of the present invention is to increase the physical scale across the reconstructed image. The microtomography device described is capable of obtaining images with 2 times more resolution elements per plane (physical scale) than conventional medical CT. Increasing the physical scale across the image is especially important in microtomography.
Another object of the present invention is to provide a reconstruction of an object on a three dimensional network of points. Instead of acquiring data in 1-20 adjacent planes (as medical tomographic devices do), the present device acquires data sufficient to reconstruct an image in more than 100 adjacent planes. This provides the ability to obtain three dimensional information about an object. As such the device described herein can be thought of as a three dimensional x-ray microscope.
Improvements in resolution and physical scale of reconstructed images come from design of the detector used to measure the transmitted intensity as well as from the computational technique used to process data. It is an object of the present invention to acquire data using an imaging electro-optic detector. This detector acquires an entire planar image or an entire linear slice of an image having large physical scale with no degradation of counting statistics in each pixel across the image. By acquiring an entire large scale planar image using an electro-optic detector, the data acquisition time and dose delivered to the sample are significantly reduced. Moreover, it is possible to construct electro-optic detectors which have significantly higher spatial resolution than the scintillation detectors used in medical CT. Since the number of data points, N, (resolution elements) acquired in a line across the image can be significantly greater than in medical CT, and data can be simultaneously acquired in multiple stacked planes, it is important to utilize data inversion techniques requiring N.sup.2 rather than N.sup.3 operations to reconstruct an image. By using data inversion techniques with N.sup.2 rather than N.sup.3 operations the time required to process the data can be decreased by a factor of more than 100 in many cases.